Chemistry Notes

UNIT-I
TECHNOLOGY OF WATER

3.1 TECHNOLOGY OF WATER
INTRODUCTION
Water is the most essential compound for all living matter on the earth.
It plays an important role in human living, industrial and agricultural purposes.
So there is no life in the earth without water.
Sources of water
The two important sources of water are (1) surface water and (2)
underground water.
Surface water
The water available on the earth’s surface is called as surface water.
Surface water includes rainwater, river water, lake water and seawater.
Underground water
Underground water includes water present between the rocks in the
earth crust, spring water, well water etc.
Reason for Depletion of underground water
The decrease in the quantum of underground water is depletion of
water. Depletion of water is mainly caused by,
1. Modernization, industrialization and population growth
2. Global warming causing excess evaporation of surface water
3. Deforestation
4. Decrease in rainfall caused by seasonal changes and
5. Effluents from the industries spoiling the ground water source.
To meet out this depletion of ground water sources, it is essential to
find alternate plans using water management techniques to recharge the
ground water sources. One of the techniques adopted is rainwater harvesting.
Rain water Harvesting
Rainwater harvesting (RWH) is a technique of capturing and storing of
rainwater (tanks, slums, lake) for useful purposes and recharging the excess
water into the ground.
The methods employed are
1. Roof top harvesting
2. Open space harvesting
Roof top harvesting
Rainwater is directly used for recharging open wells and bore wells by
directing them into it. It can also be stored in sumps or overhead tanks and
used directly.

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,Open space Harvesting
Open spaces around the buildings are used for rainwater harvesting as
follows
1. With percolation/recharge pits
2. Recharge trenches
3. Recharge wells

The recharge method used depends on the soil condition.
Advantages of rain water harvesting
1. RHW increases the ground water level.
2. It avoids the depletion of underground water.
3. Soil erosion is prevented.
4. It also prevents flooding in urban areas during rainy season.
5. It ensures the availability of water for our future generation.
Types of impurities present in water
There are three types of impurities present in water. They are
(i) Suspended and colloidal impurities.
(ii) Dissolved salts.
(iii) Microorganisms.
Types of water
There are two types of water.
They are (i) soft water and (ii) hard water.
I. Soft water readily gives lather with soap.
II. Hard water does not give lather with soap.
Hardness of water
There are two types of hardness in water. They are:
(i) Temporary Hardness: (Carbonate hardness)
It is due to the presence of calcium bicarbonate [Ca(HCO3)2] and
magnesium bicarbonate [Mg(HCO3)2].
Temporary Hardness can be removed by boiling.
(ii) Permanent Hardness: (Non-Carbonate hardness)
It is due to the presence of chloride and sulphate salts of calcium and
Magnesium. (CaCl2, CaSO4, MgCl2, MgSO4).Hence it is called as
permanent hard water.




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,Methods of expressing the Hardness.
The hardness of water can be expressed by any one of the following two
methods.
Units for measuring hardness
1. mg/litre of CaCO3
It is the number of mg CaCO3 of present in one litre of water or mg/lit
2. part per million of CaCO3
It is the number of parts by weight of CaCO3 present in million parts of
water or ppm.
1mg / litre = 1 ppm
Usually, the hardness of water is expressed in terms of calcium carbonate
equivalents.
The formula used to convert the mass of hardness producing salt to the
mass of CaCO3 equivalent is given below.
Calcium
Carbonate = Mass of salt x Molecular mass of CaCO3
Equivalents Molecular mass of salt


To prove the relation between mg/litre and ppm
Let us consider a water sample whose hardness is ‘x’ mg/litre of CaCO3.
Therefore, Mass of CaCO3 present in 1000ml of water =Y mg i.e. Mass of
CaCO3 present in 1000g of water = Y  103g
Therefore,
Mass of CaCO3 present in 106g of water = Y  103g  106


= Yx1000g.
Hence hardness of water is = Y ppm.
Therefore, 1 mg / litre = 1 ppm. Hence it is proved.
Even though CaCO3 is not a hardness producing salt and is insoluble
in water, it is used as the standard to express the hardness of water. Since
the hardness producing salts are present in traces, mass of CaCO3 equivalent
to hardness producing salt is calculated to express the hardness of a water
sample.
The formula used to convert the mass of hardness producing salt to
mass of CaCO3 is given as follows.
One molecular mass of  one molecular mass of
Hardness producing salt CaCO3
Note: Molecular masses of hardness producing salts are given below.


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, Hardness producing salt Molecular Mass
CaSO4 136
MgSO4 120
CaCl2 111
MgCl2 95
Ca (HCO3)2 162
Mg (HCO3)2 146


1. A water sample contains 48 mg of MgSO4 per 200ml of water. Calculate
the hardness in terms of CaCO3 equivalent in mg/litre of CaCO3.
One molecular mass of MgSO4 = one molecular mass of CaCO3


i.e. 120 mass of Mg SO4 = 100 mass of CaCO3


Therefore mass of 48mg of MgSO4 = 48 x 100 = 40mg of CaCO3
120
Mass of CaCO3 present in 200 ml of water = 40mg
Therefore,
Mass of CaCO3 present in 1000ml of water = 200mg
Hardness of water = 200mg/litre of CaCO3
Disadvantages of a hard water sample
 Hard water cannot be used for drinking, as it does not quench thirst.
 It cannot be used for cooking purposes.
 It cannot be used for bathing and washing purposes as it does not give
lather with soap.
 Hard water cannot be used in laboratories as it gives unwanted chemical
reactions.
 Hard water cannot be used in boilers in steam raising.
 It cannot be used in sugar and paper industries.
 Hard water cannot be used in textile and leather industries.


ESTIMATION OF HARDNESS OF WATER -EDTA METHOD
EDTA method is used to determine the hardness of a sample of water.
EDTA refers to Ethylene-diamine tetra acetic acid. This method is also called
Modern method.




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